1. Field of the Invention
The present invention relates generally to spacer grids for nuclear fuel assemblies and, more particularly, to a spacer grid for a nuclear fuel assembly which is formed from grid strips of an improved structure, thus reducing flow-induced high-frequency vibration.
2. Description of the Related Art
A nuclear reactor refers to a device that is designed to exert artificial control over the chain reaction of the nuclear fission of fissile materials and use thermal energy generated from the nuclear fission as power.
Generally, nuclear fuel that is used in a nuclear reactor is formed in such a way that enriched uranium is molded into a cylindrical pellet of a predetermined size and many pellets are inserted into fuel rods. The fuel rods constitute a nuclear fuel assembly. The nuclear fuel assembly is loaded in a core of the nuclear reactor before it is burned up in a nuclear reaction.
Referring to FIG. 1, a typical nuclear fuel assembly includes a plurality of fuel rods 10 which are located in an axial direction, a plurality of spacer grids 20 which are provided in a transverse direction of the fuel rods 10 and support the fuel rods 10, a plurality of guide thimbles 30 which are fixed to the spacer grid 20 and form a framework of the assembly, and a top nozzle 40 and a bottom nozzle 50 which respectively support upper and lower ends of the guide thimbles 30.
About 200 or more fuel rods 10 are used to form the nuclear fuel assembly. Enriched uranium is molded into a pellet of a predetermined size and installed in each fuel rod 10.
The top nozzle 40 and the bottom nozzle 50 support the upper and lower ends of the guide thimbles 30. The top nozzle 40 is provided with elastic bodies to push down an upper end of the nuclear fuel assembly, thus preventing the pressure of a coolant flowing from a lower end of the nuclear fuel assembly towards the upper end thereof from lifting up the nuclear fuel assembly. The bottom nozzle 50 supports the lower ends of the guide thimbles 30. A plurality of flow holes through which the coolant is supplied into the nuclear fuel assembly are formed in the bottom nozzle 50.
The several spacer grids 20 are arranged at predetermined intervals with respect to the axial direction of the fuel rods 10. According to the arrangement location and function, the spacer grids 20 are classified into medial spacer grids, mixing spacer grids which enhance the performance of mixing the coolant, and a protective spacer grid which filters out foreign substances.
Referring to FIG. 2, the spacer grids are commonly formed by a plurality of grid strips assembled in a lattice shape. In each spacer grid, a single fuel rod or guide thimble is disposed in each of the lattice cells.
In detail, the spacer grid 20 includes a plurality of an outer grid strip 21 which forms an outer frame of a structure, and horizontal grid strips 22 and vertical grid strips 23 which are arranged and fixed inside the outer grid strip 21 and form a lattice shape.
The fuel rods are disposed in the corresponding lattice cells 20a formed in the spacer grid 20 having the above-mentioned construction. Further, guide thimble lattice cells 20b into which the guide thimbles are inserted are formed in the spacer grid 20.
The fuel rods are assembled with the spacer grid in such a way that dimples and grid springs are provided on the grid strips that form the lattice cells so that the grid strips elastically support the fuel rods. Each guide thimble may be welded to the spacer grid or may be mechanically fixed thereto by a sleeve.
FIG. 3 is a perspective view illustrating a protective spacer grid according to a conventional technique. Only one of lattice cells formed from a plurality of grid strips is shown in this drawing.
Referring to FIG. 3, the typical spacer grid 20 includes horizontal grid strips 22 and vertical grid strips 23 which are crisscrossed and adhered to each other to form a lattice shape, thus forming lattice cells. One fuel rod is disposed in one lattice cell. Each fuel rod is supported in the corresponding lattice cell by dimples 24 which are made by bending or curving portions of the grid strips 23 and protrude from the surfaces of the grid strips 23. A grid spring may be provided to elastically support the fuel rod along with the dimples, although it is not shown in the drawing.
As such, each grid strip generally has a planar surface. The dimples or grid springs are provided to be bent or curved from the planar surface of the grid strips inwards or outwards with respect to the lattice cell. The surfaces of the grid strips that are disposed above and below the dimples and the grid springs are formed to be planar without having any specific structure.
Recently, the structure of a spacer grid which can improve the flow of a coolant that passes around fuel rods is required, for example, in such a way that mixing blades are attached to the spacer grid or the structure of a flow channel of the coolant is improved, thus making it more efficient to transfer heat from the fuel rods to the coolant.
However, such methods for promoting heat transfer may cause flow-induced vibration which creates greater turbulence in the coolant that flows around the fuel rods, thus vibrating the fuel rods.
The flow-induced vibration of the fuel rods causes the fuel rods to slip out of the grid spring or dimples, causing a fretting phenomenon in which partial abrasion occurs on the contact surface between the fuel rods and the grid spring or dimples, thus gradually damaging the fuel rods.
For example, a spacer grid for preventing the fretting of fuel rods was proposed in Korean Patent Registration No. 10-0932436 (date: Dec. 9, 2009), which improves a contact structure between the fuel rods and the grid springs to prevent flow-induced vibration from causing axial or lateral vibration of the fuel rods.
As such, different kinds of means for reducing flow-induced vibration of the coolant in the spacer grid of the nuclear fuel assembly have been devised. The present invention is to provide a structure of a spacer grid that can reduce flow-induced vibration.